1. Field of the Invention.
This invention relates to microminiature tips and, more particularly, to microminiature tips formed using semiconductor integrated circuit fabrication techniques.
2. Prior Art.
An atomic force microscope (AFM) scans over the surface of a sample in two different modes of operation. In one mode, the contacting mode, a sharp tip is mounted on the end of a cantilever and the tip rides on the surface of a sample with an extremely light tracking force, on the order of 10.sup.-5 to 10.sup.-10 N. In the contacting mode of operation, profiles of the surface topology are obtained with extremely high resolution. Images showing the position of individual atoms are routinely obtained. In the other mode, the tip is held a short distance, on the order of 5 to 500 Angstroms, from the surface of a sample and is deflected by various forces between the sample and the tip, such forces include electrostatic, magnetic, and van der Waals forces.
Several methods of detecting the deflection of the cantilever are available with subangstrom sensitivity, including vacuum tunneling, optical interferometry, optical beam deflection, and capacitive techniques. However, fabrication of a readily reproducible cantilever stylus assembly has been a limiting factor on use of AFM and other forms of microscopy such as scanning tunneling microscopy.
The technical requirements for the cantilever-and-tip assembly include a number of different factors. A low force constant for the cantilever is desirable so that reasonable values of deflection are obtained with relatively small deflection forces. Typical values are 0.01-1000 N/m. A mechanical resonant frequency for the cantilever which is greater than 10 kHz is desirable to increase image tracking speed and to reduce sensitivity to ambient vibrations. Low force constants and high resonant frequencies are obtained by minimizing the mass of the cantilever.
When optical beam deflection is used to detect deflection of the cantilever, deflection sensitivity is inversely proportional to the length of the cantilever. Therefore a cantilever length of less than 1 mm. is desirable.
For certain types of deflection sensing, a high mechanical Q is desirable and is achieved by using amorphous or single crystal thin films for fabrication of the cantilever.
In many applications, it is desirable that the cantilever flex in only one direction and have high lateral stiffness. This can be obtained by using a geometry such as a V-shape which has two arms obliquely extending and meeting at a point at which the tip is mounted.
It is often required that a conductive electrode or reflective spot be located on the side of the cantilever opposite the tip. This is obtained by fabricating the cantilever from metal or depositing a conductor on certain portions of the cantilever to serve as a conductor or reflector.
Finally, a sharp tip, that is, a protruding tip with a tip radius less than 500 Angstroms and which may terminate in a single atom, is desired to provide good lateral resolution. The sharper the tip, the higher the resolution, especially when operating in the contact mode. This requirement has traditionally been one of the most difficult to obtain in a reproducible manner. Typically, in the prior art, tips were fabricated by hand using fabrication and bonding techniques which were time consuming and which produced non-uniformly performing tips.
In the prior art, cantilever arms were constructed by hand from fine wires. One way of obtaining a tip portion was to etch a wire to a point and to bend the point to extend perpendicularly from the wire. Another way to obtain a tip was to glue a tiny diamond fragment in place at the end of a cantilever. Prior art cantilevers fabricated using photolithographic techniques did not have integrally-formed sharp protruding tips. A rather dull tip was effectively obtained by using a corner of the microfabricated cantilever itself as a tip. Alternatively, a diamond fragment was glued by hand to the end of a microfabricated cantilever. The cantilever assembly of an AFM is relatively fragile and is virtually impossible to clean when it is contaminated by material from the surface being scanned so that frequent replacement is required.
Background information on well known prior art fabrication methods used in this invention can be found in the following references: Information on wet anisotropic Si etching and macromachining in general is described by K. E. Petersen, Proc. IEEE 70, 420 (1982). Isotropic and anisotropic dry plasma etching of Si and other materials is discussed in the book "Silicon Processing for the VLSI Era" by Wolf and Tauber. Inert ion etching, including ion milling, and many other aspects of microfabrication are discussed in the book "The Physics of Microfabrication" by Brodie and Muray. The electromigration phenomena exploited in the construction of dielectric tips with conducting metallic points is discussed in the book "VLSI Fabrication Principles" by Ghandhi. Information on the need for insulated STM tips is discussed by M. M. Dovek et al. in "Molecular Phenomena at Electrode Surfaces," ACS Symposia Series, edited by M. P. Soriga (in press).
Extremely high resolution profiles of surface topography can be obtained in the contacting mode of operation; in fact, images showing the positions of individual surface atoms are routinely obtained with the AFM. Therefore, a reproducible method for fabricating tips is a significant contribution to AFM technology.